Interpreting SCUBA-2 map Quality Arrays

SCUBA-2 maps created using "AST-masking", such as is done by  dimmconfig_bright_extended, will contain a Quality array indicating the background pixels that were masked (i.e. forced to zero). In itself this is fairly simple - background pixels have a non-zero Quality value and source pixels have a Quality value of zero. However, it is also possible to have independent masks for the FLT and COM models, in addition to the AST mask. For instance, if the "auto-FLT masking" scheme is used the Quality array contains both a FLT and an AST mask and so some care needs to be used when interpreting the Quality array.

Each value in the Quality array is restricted to taking integer values between 0 and 255, and so can be thought of as 8 separate bits. Each "bit plane" within the Quality array holds a single mask - AST, FLT or COM.   The showqual command can be used to find out which mask is held by which bit plane:

% kappa
% showqual fred.sdf
   AST (bit 1) - "Set iff AST model is zeroed at the output pixel"
   FLT (bit 2) - "Set iff FLT model is blanked at the output pixel"

This means that the AST mask is stored in bit 1 (the least significant bit), the FLT mask is stored in bit 2, and there is no COM mask. Note, if a map was produced using FLT masking but no AST masking, then the FLT mask would be stored in bit 1.

The decimal integer value of any element of the Quality array is equal to the binary value formed from the bits listed by showqual. So in the above case the maximum Quality value is 3 (the decimal equivalent of binary "11" - both bits set). Remembering that a bit is set (i.e. is 1) for background pixels and cleared (i.e. is 0) for source pixels, it follows that the four possible decimal Quality values in the above case (0-3) are:

0 - neither bit set, so the pixel is inside both the AST and the FLT mask (a source pixel).

1 - bit 1 set but not bit 2, so the pixel is outside the AST mask but inside the FLT mask (a border-line pixel).

2 - bit 2 set but not bit 1, so the pixel is inside the AST mask but outside the FLT mask (a border-line pixel).

3 - both bits set, so the pixel is inside neither mask (a background pixel).

In the following screen shot of a Quality array displayed using GAIA, the black areas have value zero and are thus inside both masks, the dark brown areas have value 2 and are thus inside the AST mask but outside the FLT mask. The light brown areas have value 3 and are inside neither mask. In this particular case, there are no  areas with a quality value of 1, so the FLT mask is contained entirely within the AST mask.

There are several commands within kappa that manipulate Quality arrays in various ways. For instance, the setbb command allows pixel data values to be set bad if the associated Quality value has a specified collection of set bits. Thus:

% setbb fred 1

will set all pixels bad except for those inside the AST mask. Likewise, 

% setbb fred 2

will set all pixels bad except for those inside the FLT mask.  Note, the change made by setbb is temporary - it can be undone by doing:

% setbb fred 0

To display the fred.sdf map and then overlay the AST mask in blue and the FLT mask in red, do:

% display fred mode=perc percentiles=\[2,98\]
% setbb fred 1
% contour fred clear=no mode=good labpos=! style='colour=blue'
% setbb fred 2
% contour fred clear=no mode=good labpos=! style='colour=red'
% setbb fred 0